Smart window changes color with weather

Weather could power the next generation of smart windows. Researchers have created glass that tints by harvesting energy from wind and precipitation. The approach offers an alternative to other smart windows powered by batteries, solar panels, and even standard power outlets.

“The innovation … represents a new kind of renewable energy source,” says Liming Dai, a nanomaterials engineer at Case Western Reserve University in Cleveland, Ohio, who was not involved in the research.

Smart glass, which changes its properties to block out light or heat, has been around for decades. Common examples include glare-fighting rearview car mirrors and windows that change color for energy savings and privacy. But many are expensive, and people are still looking for eco-friendly ways to power the windows. Batteries and plug-in outlets aren’t ostensibly “green,” whereas embedded solar panels can cloud or obscure segments of the glass.

The new glass uses nanosized generators powered by triboelectrics—the static electricity produced by friction when two materials touch. When activated, the generators, which rest in two layers atop a single pane of glass, create an electric current that tints the clear window a dark shade of blue.

The outermost layer of generators harvests static energy from rain. When a raindrop falls from a cloud, the contact between the water and the air creates a positive charge within the droplet. When the droplet strikes the glass, which is coated with nanoscopic pyramids made from a negatively charged silicone material called polydimethylsiloxane, it creates an electric current.

The second layer of nanogenerators lies just beneath the first and harvests energy from the wind. This layer consists of two sheets of charged, see-through plastic that are separated by nanoscopic spring coils. As wind pushes against the window, the springs compress and create an electric current as the charged plates of plastic approach each other.

In experiments, the glass produced up to 130 milliwatts per square meter, enough to power a pacemaker or a smart phone while it’s asleep, the team reported online last month in ACS Nano. This output might suit many applications, such as being a power source for home or office electronics, says co-developer and nanoscientist Zhong Wang of the Georgia Institute of Technology in Atlanta. Since their first project in 2012, a light-up sidewalk powered by footsteps, he and his colleagues have miniaturized their generators to create everything from self-cleaning keyboards to sensors for security systems.

But Wang and colleagues still have more work to do before this smart glass is ready for commercialization. Now, the glass has no way to store the energy it creates. To solve this problem, Dai says, transparent supercapacitors could be laced into the glass without decreasing visibility.

For now, the team wants to boost the energy efficiency of their nanogenerators. These tiny power plants can convert about 60% of the mechanical energy that they encounter into electricity. “The output power is a constant goal," Wang says. “Free energy surrounds us, and anything can happen if you harness it.”